The present invention relates generally to semiconductor switching devices, and more particularly, to very fast field-effect transistors.
Field effect transistors have been known for many years and have been found to be especially suitable for high frequency switching operations. In this regard, such state of the art FET's as the Fujitsu HEMT have switching times on the order of 10-20 picoseconds. These high switching times are the result of forming the FET channel in GaAs formed on an adjacent substrate of AlGaAs. The GaAs/AlGaAs interface is an epitaxial heterojunction interface which allows the carriers in the GaAs channel to move with low scattering rates due to the high geometric atomic order of that interface. The AlGaAs substrate is doped in order to provide carriers for the FET channel. However, a problem when using such donor substrates is that when the charged carriers propagate into the FET channel, the donor atoms remaining in the substrate have a charge of the opposite type to that of the donated carrier. In the case of an n-doped substrate, the corresponding donor atoms remaining in the substrate will all have a positive charge. This positive charge in the substrate atoms adjacent to the FET channel causes the electrons flowing in that channel to scatter. This scattering caused by the charge on the substrate donors can be avoided by the modulation doping of the substrate layers immediately adjacent to the FET channel. This modulation doping simply comprises forming the last 50-100 angstroms of the substrate with neutral or non-doped material. In essence, the donor scattering is almost totally avoided by buffering the charged donor atoms in the substrate with an undoped region.
Although the forgoing modulation doped state of-the-art FET has a fast switching time, the FET channel is relatively long, leading to an inherent speed limitation in the device. Furthermore, the dynamic range of the device is limited by the fixed pinning of the energy bands at the GaAs/AlGaAs interface. Accordingly the transconductance, i.e. the change of drain current caused by a change in gate voltage, is inherently limited because it depends on the ohmic resistance between the source and the gate region of the FET. This ohmic resistance, in turn, is directly related to the physical length of the channel path. Attempts to reduce this channel length by bringing the source and the drain physically closer have resulted in significantly increased capacitive coupling between those electrodes. Such coupling results in the shorting out of the gate voltage at high switching rates.